This invention relates, in general, to a method of forming an electrical interconnect, and more particularly, a method of forming an interconnect bump.
In the semiconductor industry, electrical contact is made to a semiconductor die by forming a bump or raised metal contact on the surface of the semiconductor die in order to facilitate electrical contact to another surface, such as a printed circuit board, a package, or lead.
In the past, this was accomplished by using a dry resist laminate, typically called a solder mask, to mask out certain portions of a printed circuit board on which the bump is not formed. The bumps are formed using solder paste which is screen printed. The dry resist laminate has been used in printed circuit board applications in order to form large interconnect bumps greater than 100 microns.
The disadvantage of using a solder mask is that small geometries are not attainable due to the minimum thickness of the dry photoresist laminate and the minimum geometries which may be printed on the dry photoresist laminate. The dry resist laminate has a thickness in the range of 75-150 microns. In printed circuit board applications, this has not been a problem because there has been no need to attain small geometry electrical contacts.
Dry resist laminates have been used to form electroplated interconnect bumps on semiconductor wafers or chips. However, these dry resist laminates are difficult to apply to semiconductor wafers. Overall, the dry resist laminates are not suitable for use in a semiconductor wafer manufacturing environment where bump geometries must be less than 50 microns and where high throughput, reduced cycle time and reduced scrap is desirable.
Another method used in the past is screen or stencil printing using a metal stencil or screen as a mask. The metal mask was used to cover portions of a semiconductor wafer so that either a solderable material was evaporated or a conductive polymer or solder paste was forced through the openings of the metal mask. One disadvantage of screen or stencil printing using a metal mask is that this technology is limited to forming bumps which are 50 microns or greater in diameter or width and a 100 micron pitch because of the difficulty of making small openings of this size in the metal mask and the accuracy of aligning the openings of the metal mask to the semiconductor wafer bond pad. In addition, this process of evaporating bumps using a metal stencil is very expensive.
There is a need to form electrical contact bumps having diameters of less than 50 microns. In addition, it would be desirable to provide a method of forming such electrical contacts at low cost and high throughput.